The United States has warned its citizens in China to stay alert, after a government employee suffered brain damage in a suspect sonic attack.

The US State Department issued the health alert on Wednesday, when the employee in the southern Chinese city of Guangzhou complained of "subtle and vague, but abnormal, sensations of sound and pressure."

The department noted that it was not aware of any other cases inside or outside the diplomatic community but advised American citizens across China to consult a doctor if they experienced any symptoms.

"The Chinese government has assured us they are also investigating and taking appropriate measures," said Jinnie Lee, a spokeswoman for the US Embassy in Beijing, adding the department was taking the incident "very seriously."

The statement also asked citizens to move to other place in case they started to "experience any unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises."

According to Lee, the employee reported the symptoms from late 2017 to April 2018 and was diagnosed with mild traumatic brain injury after undergoing medical tests in the US.

The alert was similar to a slew of warnings the US government issued to its employees in Cuba, after American and Canadian diplomats stationed in the country were targeted in a rash of mysterious "sonic attacks."

Unproven sonic attacks ‘economic weapon’ against Cuba

The allegations of “sonic attacks” in Havana against Western diplomats continue to be rejected by the scientific community.

The issue become so severe that Washington withdrew a large number of its embassy staff from the country last year after diplomats complained of hearing loss, dizziness, tinnitus, visual difficulties, headaches, fatigue and other symptoms.

The American Foreign Service Association said back then that the diplomats were diagnosed with "mild traumatic brain injury and permanent hearing loss, with such additional symptoms as loss of balance, severe headaches, cognitive disruption, and brain swelling."

Denying that it had anything to do with any possible attacks, the Cuban government launched a broad investigation into the claims, enlisting some 2,000 security officials and experts.

US President Donald Trump insisted that Havana was responsible for the physical harm done to the American diplomats.